1. Field of the Invention
The present invention relates to ternary mixed lithium oxides, in particular to doped lithium manganese oxides having a spinel-type crystal structure. The present invention further relates to a process for preparing ternary mixed lithium oxides and to their use, in particular as a cathode material in secondary lithium batteries.
2. Description of the Related Art
Because of the high energy density rechargeable lithium batteries can achieve and because of their low weight, there is a growing need for rechargeable lithium batteries for a wide range of applications, both as electric accumulators for portable electronic equipment such as e.g. video cameras or laptop computers, and, in future, as traction batteries.
The use of elemental lithium as an anode material is known to result in inadequate cycle stability of the battery, owing to dendrite formation when the lithium is dissolved and redeposited, and in a considerable safety risk.
An attempt to overcome these problems led to the development of the so-called "lithium ion" or "rocking chair" battery. Its functional principle is based on the use of electrode materials which are able to intercalate lithium reversibly, both for the anode and the cathode. At present, a lithium-containing carbon compound is usually employed as the anode and a lithium-containing ternary oxide system is used as the cathode.
So as to be able to achieve the highest possible energy densities, cathode materials are preferably employed which are able to intercalate lithium at potentials of between 3 and 4 V vs. Li/Li.sup.+. Among the most promising materials meeting these requirements are ternary lithium compounds on the basis of oxides of cobalt, nickel and manganese.
Among the lithium manganates, the spinel LiMn.sub.2 O.sub.4 and compounds related to spinels (e.g. Li.sub.4 Mn.sub.4 O.sub.9 and Li.sub.4 Mn.sub.5 O.sub.12) exhibit the best characteristics as cathode materials (Thackeray et al., Mater. Res. Bull. 18, 561 (1983)).
As a rule, these materials are prepared by means of solid-state reactions by the corresponding oxides and/or carbonates being mixed together and the mixtures being heated to elevated temperatures (U.S. Pat. No. 4,980,251; J. Electrochem. Soc., Vol. 138, No. 10, 1991, pp. 2859-2864; and JP-A-03-283 356). As a rule, this gives rise to stoichiometric spinels whose cycle lifetime is usually inadequate.
According to the prior art, the battery-relevant characteristics of these cathode materials, in particular those of the manganese oxides, depend critically on the preparation parameters and especially on the reaction temperature.
Advantages are offered by a synthesis process at the lowest possible temperatures, which results in materials having a uniform particle distribution. According to U.S. Pat. No. 5,135,732 and H. Huang, J. Electrochem. Soc. 141 (1994) L76, the relevant lithium compounds on the basis of cobalt oxide or manganese oxide can also be prepared by a low-temperature synthesis via acetate precursors.
A partial reduction of permanganate solutions likewise allows the synthesis of lithium manganese oxides at low temperatures (Bach et al., J. Solid State Chem. 88 (1988), 325).
The pure ternary oxides have only a limited cycle lifetime, and a continuous deterioration of the capacity is often observed as the number of cycles increases, which can be ascribed to changes and defects in the lattice structure of the host oxide as lithium cations are being incorporated and removed.
With all the low-temperature synthesis processes known hitherto, which give rise to cathode materials having spinel structures or spinel-related structures, the product is a pure ternary oxide or a ternary oxide containing admixtures of carbon which is not incorporated in the lattice.